James R. Holmquist

A Database and Synthesis of Northern Peatland Soil Properties and Holocene Carbon and Nitrogen Accumulation

Here, we present results from the most comprehensive compilation of Holocene peat soil properties with associated carbon and nitrogen accumulation rates for northern peatlands. Our database consists of 268 peat cores from 215 sites located north of 45°N. It encompasses regions within which peat carbon data have only recently become available, such as the West Siberia Lowlands, the Hudson Bay Lowlands, Kamchatka in Far East Russia, and the Tibetan Plateau. For all northern peatlands, carbon content in organic matter was estimated at 42 ± 3% (standard deviation) for Sphagnum peat, 51 ± 2% for non-Sphagnum peat, and at 49 ± 2% overall. Dry bulk density averaged 0.12 ± 0.07 g/cm3, organic matter bulk density averaged 0.11 ± 0.05 g/cm3, and total carbon content in peat averaged 47 ± 6%. In general, large differences were found between Sphagnum and non-Sphagnum peat types in terms of peat properties. Time-weighted peat carbon accumulation rates averaged 23 ± 2 (standard error of mean) g C/m2/yr during the Holocene on the basis of 151 peat cores from 127 sites, with the highest rates of carbon accumulation (25–28 g C/m2/yr) recorded during the early Holocene when the climate was warmer than the present. Furthermore, we estimate the northern peatland carbon and nitrogen pools at 436 and 10 gigatons, respectively. The database is publicly available at https://peatlands.lehigh.edu.

Peatland Initiation, Carbon Accumulation, and 2 ka Depth in the James Bay Lowland and Adjacent Regions

Abstract Peatlands surrounding Hudson and James Bays form the second largest peatland complex in the world and contain major stores of soil carbon (C). This study utilized a transect of eight ombrotrophic peat cores from remote regions of central and northern Ontario to quantify the magnitude and rate of C accumulation since peatland initiation and for the past 2000 calendar years before present (2 ka). These new data were supplemented by 17 millennially resolved chronologies from a literature review covering the Boreal Shield, Hudson Plains, and Taiga Shield bordering Hudson and James Bays. Peatlands initiated in central and northern Ontario by 7.8 ka following deglaciation and isostatic emergence of northern areas to above sea level. Total C accumulated since inception averaged 109.7 ± (std. dev.) 36.2 kg C m-2. Approximately 40% of total soil C has accumulated since 2 ka at an average apparent rate of 20.2 ± 6.9 g C m-2 yr-1. The 2 ka depths correlate significantly and positively with modern gridded climate estimates for mean annual precipitation, mean annual air temperature, growing degree-days > 0 °C, and photosynthetically active radiation integrated over days > 0 °C. There are significantly shallower depths in permafrost peatlands. Vertical peat accumulation was likely constrained by temperature, growing season length, and photosynthetically active radiation over the last 2 ka in the Hudson Bay Lowlands and surrounding regions.

Temperature, oxygen, and vegetation controls on decomposition in a James Bay peatland

The biochemical composition of a peat core from James Bay Lowland, Canada, was used to assess the extent of peat decomposition and diagenetic alteration. Our goal was to identify environmental controls on peat decomposition, particularly its sensitivity to naturally occurring changes in temperature, oxygen exposure time, and vegetation. All three varied substantially during the last 7000 years, providing a natural experiment for evaluating their effects on decomposition. The bottom 50 cm of the core formed during the Holocene Climatic Optimum (~7000–4000 years B.P.), when mean annual air temperature was likely 1–2°C warmer than present. A reconstruction of the water table level using testate amoebae indicated oxygen exposure time was highest in the subsequent upper portion of the core between 150 and 225 cm depth (from ~2560 to 4210 years B.P.) and the plant community shifted from mostly Sphagnum to vascular plant dominance. Several independent biochemical indices indicated that decomposition was greatest in this interval. Hydrolysable amino acid yields, hydroxyproline yields, and acid:aldehyde ratios of syringyl lignin phenols were higher, while hydrolysable neutral sugar yields and carbon:nitrogen ratios were lower in this zone of both vascular plant vegetation and elevated oxygen exposure time. Thus, peat formed during the Holocene Climatic Optimum did not appear to be more extensively decomposed than peat formed during subsequent cooler periods. Comparison with a core from the West Siberian Lowland, Russia, indicates that oxygen exposure time and vegetation are both important controls on decomposition, while temperature appears to be of secondary importance. The low apparent sensitivity of decomposition to temperature is consistent with recent observations of a positive correlation between peat accumulation rates and mean annual temperature, suggesting that contemporary warming could enhance peatland carbon sequestration, although this could be offset by an increasing contribution of vascular plants to the vegetation.

Peatland succession and long-term apparent carbon accumulation in central and northern Ontario, Canada

Despite their importance as globally significant carbon (C) stores, basic knowledge of post-glacial peatland history and C accumulation are lacking for the Canadian Boreal Shield and James Bay Lowland (JBL) of central and northern Ontario, Canada. Radiocarbon dates, plant macrofossil analysis, and soil C estimates from an eight-core transect of the JBL and surrounding regions are used to reconstruct the timings and patterns of fen to bog transitions, and the ranges and patterns of long-term apparent rate of C accumulation (LARCA). Peatland initiation lagged the retreat of the Laurentide ice sheet, the drainage of glacial lakes, and isostatic uplift by 810–6050 years. Transition from Carex-dominated fen to Sphagnum-dominated bog had a median timing of 3500 years following peatland establishment and ranged from 640 to 6970 years. LARCA was variable geographically and over time with median values ranging from 13.4 to 31.6 g C/m2/yr. LARCA anomalies were generally high ~6.1 kyr (kyr = 1000 calibrated years before present (cal. yr BP)) for southern sites, and ~2.5 kyr for the most northern sites, and may be associated with elevated moisture as inferred from a brief review of regional proxy reconstructions. Some sites displayed high LARCA anomalies, changes in plant ecology, or southern site initiation, which may have also been driven by a moist Hypsithermal Period occurring ~4.5 kyr. LARCA increases were not generally associated with high-temperature anomalies during the warm ‘Medieval Climate Anomaly’ compared with the cooler ‘Little Ice Age’; however, there is evidence that the establishment of modern permafrost during the late-Holocene negatively affected C accumulation.

Prolonged California aridity linked to climate warming and Pacific sea surface temperature

California has experienced a dry 21st century capped by severe drought from 2012 through 2015 prompting questions about hydroclimatic sensitivity to anthropogenic climate change and implications for the future. We address these questions using a Holocene lake sediment record of hydrologic change from the Sierra Nevada Mountains coupled with marine sediment records from the Pacific. These data provide evidence of a persistent relationship between past climate warming, Pacific sea surface temperature (SST) shifts and centennial to millennial episodes of California aridity. The link is most evident during the thermal-maximum of the mid-Holocene (~8 to 3 ka; ka = 1,000 calendar years before present) and during the Medieval Climate Anomaly (MCA) (~1 ka to 0.7 ka). In both cases, climate warming corresponded with cooling of the eastern tropical Pacific despite differences in the factors producing increased radiative forcing. The magnitude of prolonged eastern Pacific cooling was modest, similar to observed La Niña excursions of 1o to 2 °C. Given differences with current radiative forcing it remains uncertain if the Pacific will react in a similar manner in the 21st century, but should it follow apparent past behavior more intense and prolonged aridity in California would result.

U.S. Pacific coastal wetland resilience and vulnerability to sea-level rise

A comprehensive field and modeling study indicates vulnerability of tidal wetlands to sea-level rise on the U.S. Pacific coast. We used a first-of-its-kind comprehensive scenario approach to evaluate both the vertical and horizontal response of tidal wetlands to projected changes in the rate of sea-level rise (SLR) across 14 estuaries along the Pacific coast of the continental United States. Throughout the U.S. Pacific region, we found that tidal wetlands are highly vulnerable to end-of-century submergence, with resulting extensive loss of habitat. Using higher-range SLR scenarios, all high and middle marsh habitats were lost, with 83% of current tidal wetlands transitioning to unvegetated habitats by 2110. The wetland area lost was greater in California and Oregon (100%) but still severe in Washington, with 68% submerged by the end of the century. The only wetland habitat remaining at the end of the century was low marsh under higher-range SLR rates. Tidal wetland loss was also likely under more conservative SLR scenarios, including loss of 95% of high marsh and 60% of middle marsh habitats by the end of the century. Horizontal migration of most wetlands was constrained by coastal development or steep topography, with just two wetland sites having sufficient upland space for migration and the possibility for nearly 1:1 replacement, making SLR threats particularly high in this region and generally undocumented. With low vertical accretion rates and little upland migration space, Pacific coast tidal wetlands are at imminent risk of submergence with projected rates of rapid SLR.

Marine Radiocarbon Reservoir Values in Southern California Estuaries: Interspecies, Latitudinal, and Interannual Variability

Many studies use radiocarbon dates on estuarine shell material to build age-depth models of sediment accumulation in estuaries in California, USA. Marine 14 C ages are typically older than dates from contemporaneous terrestrial carbon and local offsets (∆R) from the global average marine offset need to be calculated to ensure the accuracy of calibrated dates. We used accelerator mass spectrometry (AMS) 14 C dating on 40 pre-1950 salt marsh snail and clam shells previously collected from four California estuaries. The average ∆R and standard deviation of 217 ± 129 14 C yr is consistent with previous calculations using mixed estuarine and marine samples, although the standard deviation and resulting age uncertainty was higher for our estuarine calculations than previous studies. There was a slight but significant difference ( p = 0.024) in ∆R between epifaunal snails (∆R = 171 ± 154 14 C yr) and infaunal clams (∆R = 263 ± 77 14 C yr), as well as between samples from individual estuaries. However, a closer examination of the data shows that even for the same species, at the same estuary, ∆R can vary as much as ~500 14 C yr. In some cases, the bulk of this variation occurs between samples collected by different collectors at different times, potentially indicating time dependence in carbon sources and ∆R variation. These variations could also be attributed to differences in collection location within a single estuary and resulting spatial differences in carbon sources. Intertidal specimens located in the high marsh may have lower ∆R than fully marine counterparts because of increased terrestrial 14 C input. The large variations in ∆R here highlight the need for conservative chronological interpretations, as well as the assumption of wide uncertainties, when dating samples from estuarine sources. DOI: 10.2458/azu_rc.57.18389

Long-term successional changes in peatlands of the Hudson Bay Lowlands, Canada inferred from the ecological dynamics of multiple proxies

Peatlands in northern Ontario, Canada, archive multiple biological indicators, including macrofossils, algae, testate amoebae, and pollen. These proxies can provide insights concerning the timing and nature of long-term climatic and environmental changes. The Hudson Bay Lowlands (HBL) of central Canada contain one of Earth’s largest continuous peatland complexes, and thus comprehensive spatial and temporal studies are needed to understand the implications of climate change on carbon cycling. Diatom assemblages were examined in three cores retrieved from ombrotrophic bogs across two Canadian terrestrial ecozones. Comparisons were made with testate amoebae and macrofossil data previously analyzed from these cores, as well as with regional pollen records from surrounding peatlands. From ~2000 to ~600 cal. BP, changes in diatom composition likely reflect hydrosere succession within the peatland, including fluctuations in connectivity to the water table and pH changes. From ~600 cal. BP to present, the synchronous timing of changes in diatoms and testate amoebae are tracking drying conditions and subsequent microhabitat variations that occur within bogs. It is possible that diatoms are tracking subtle changes in the stability of peat microforms including bog hollows and hummocks, highlighting their sensitivity to small chemical change, whereas testate amoebae are tracking the lowering of a peatland water table and subsequent drying of the peatland. The use of multiple proxies provides a more holistic approach to interpreting past ecological succession and responses to climate within peatlands. When present and well preserved, diatoms can be applied to track drying conditions in bogs, in terms of both hydrosere succession and present climatic change.

Peatland paleohydrology in the southern West Siberian Lowlands: Comparison of multiple testate amoeba transfer functions, sites, and Sphagnum δ13C values

A 2700-year-old peat core from the southern West Siberian Lowlands was used to reconstruct past water-table depth using testate amoeba analysis and to compare hydrological changes with temperature variations associated with the Medieval Climate Anomaly, ‘Little Ice Age’, and 20th-century warming. The robustness of water-table results was assessed using comparisons of four separate transfer functions, a testate amoeba reconstruction from an additional site in southern West Siberia, and an independent hydrological proxy of the δ13C values of Sphagnum remains from the same core. The paleohydrology results were robust in that (1) all four transfer functions returned similar results, (2) both peatland sites displayed very similar water-table fluctuations despite their distance from each other, and (3) Sphagnum δ13C values showed similar overall changes as the testate amoeba–inferred hydrology, but at a coarser temporal resolution. When comparing reconstructed hydrology in southern West Siberia to Northern Hemisphere temperatures, we found that during most of the record warmer time intervals tended to be wet locally and cooler time intervals tended to be dry including during the Medieval Climate Anomaly (~1150–650 cal. BP). This pairing continued until the ‘Little Ice Age’ (~650–100 cal. BP) when conditions became cool and wet, and recently, conditions have become warm again, but unlike the earlier wet interval, the peatlands have dried. Drier conditions shown by the water-table depth reconstruction suggest that future warming may continue the drying of southern peatland surfaces in the West Siberian Lowlands and may promote peat carbon respiration.

Carbon Budget of Tidal Wetlands, Estuaries, and Shelf Waters of Eastern North America

Carbon cycling in the coastal zone affects global carbon budgets and is critical for understanding the urgent issues of hypoxia, acidification, and tidal wetland loss. However, there are no regional carbon budgets spanning the three main ecosystems in coastal waters: tidal wetlands, estuaries, and shelf waters. Here, we construct such a budget for Eastern North America using historical data, empirical models, remote-sensing algorithms, and process-based models. Considering the net fluxes of total carbon at the domain boundaries, 59 ± 12% (± 2 standard errors) of the carbon entering is from rivers and 41 ± 12% is from the atmosphere, while 80 ± 9% of the carbon leaving is exported to the open ocean and 20 ± 9% is buried. Net lateral carbon transfers between the three main ecosystem types are comparable to fluxes at the domain boundaries. Each ecosystem type contributes substantially to exchange with the atmosphere, with CO2 uptake split evenly between tidal wetlands and shelf waters, and estuarine CO2 outgassing offsetting half of the uptake. Similarly, burial is about equal in tidal wetlands and shelf waters, while estuaries play a smaller but still substantial role. The importance of tidal wetlands and estuaries in the overall budget is remarkable given that they respectively make up only 2.4 and 8.9% of the study domain area. This study shows that coastal carbon budgets should explicitly include tidal wetlands, estuaries, shelf waters and the linkages between them; ignoring any of them may produce a biased picture of coastal carbon cycling.